As known in the related art, in an Ethernet-based network, a semiconductor optical amplifier (SOA) of a central office (CO) is an amplifier having a bidirectional amplifying characteristic, in which holes and electrons are rapidly recombined When different modulating signals are input to the SOA at the same time, the SOA has non-linear mutual gain saturation, which causes distortion to occur between optical signals. That is, when two or more different optical signals are input to the SOA and are then amplified in a saturated region, crosstalk occurs in the gain between the optical signals and the phases of the optical signals, which causes mutual gain saturation.
At that time, signal components of two optical signals are distorted by the mutual gain saturation in the SOA, and thus the two optical signals do not include the original information any longer. That is, the distorted optical signals reach an upstream-side optical line terminal (OLT) of the CO by the bidirectional amplifying characteristic of the SOA and are read as error signals caused by the distortion between different subscriber signals. In addition, the optical signals transmitted in the downstream direction by the SOA allow an inexpensive optical detector, such as an optical network unit (ONU), to check distortion occurring in a common line.
FIG. 1 is a block diagram illustrating a general optical network. As shown in FIG. 1, a downstream optical signal transmitted from an OLT 7 of a CO 5 is divided by a power splitter (PS) 3 position in a remote site, and the divided optical signals reach a plurality of optical network units (ONUs) 1-1, 1-2, . . . , 1-n that are positioned on the downstream side. The optical signal from the individual ONU is transmitted to OLT through a common network by using a remote node (RN) positioned in a remote site. At that time, a plurality of subscribers cannot use the common network at the same time, and thus a technique for controlling it, such as a carrier sense multiple access/collision detection (CSMA/CD) technique, is required. Therefore, the CSMA/CD technique enables a plurality of subscribers who want to transmit signal in the downstream direction to provide an upstream service to the CO 5.
More specifically, as shown in FIG. 1, in an optical network, the OLT 7 of the CO 5 is composed of a transmitting terminal for transmitting a downstream optical signal and a receiving terminal for receiving an upstream signal, and the downstream signals are simultaneously transmitted to the plurality of ONUs 1-1, 1-2, . . . , 1-n through the PS 3 of the remote node over the common network. In addition, upstream signals generated by the plurality of ONUs 1-1, 1-2, . . . , 1-n are combined by the PS 3 and are then transmitted to the OLT 7 of the CO 5 over the common network. If the distance from the OLT 7 to the plurality of ONUs 1-1, 1-2, . . . , I-n is large, an optical amplifier (OA) may be provided in the front state of the receiving terminal of the OLT 7 in order to compensate for loss due to the long distance. In this case, a Fabry-Perot laser or a distributed feedback laser may be used as a light source for each of the plurality of ONUs 1-1, 1-2, . . . , 1-n. Preferably, an inexpensive light source, such as the Fabry-Perot laser, is used to reduce the cost of the optical network.
In the above-mentioned configuration, when a plurality of subscribers transmit optical signals at the same time and distortion occurs in the transmitted optical signals, CSMA/CD can be used to control the distortion occurring in the optical signals. CSMA/CD is performed by an electric switching method using media access control (MAC). In order to apply the method to an optical network, optical signals are converted into electric signals and are then analyzed in network layer No. 2, and it is checked whether distortion occurs in the common network due to a plurality of subscribers. However, an additional circuit is needed to check whether the distortion occurs, and the check is performed in an electric area, resulting in a time delay. Therefore, the method is not suitable for a high-speed optical Ethernet environment in which a large amount of data is transmitted at high speed.
In order to solve the above-mentioned problem, a technique for enabling a PS positioned in a remote site to monitor the usage state of all optical network units with respect to a common network through a loop-back in an optical method has been proposed. However, the technique is excellent in transmission efficiency, but cannot solve the above-mentioned problem when distances between the PS 3 and subscribers are different from each other or when optical outputs from the ONUs are different from each other. In addition, collision is detected on the basis of the intensity of an optical return signal. Therefore, in order to detect the collision, the output optical powers of all the subscribers should be equal to each other at the input terminal of the PS 3. However, when input optical power varies due to a change in an optical path, an error in detecting distortion occurs.
As another method, subscribers use different wavelengths to physically prevent the distortion of optical signals. However, in this case, service providers need to have additional lasers for an optical network having different wavelengths, and an arrayed waveguide grating router (AWGR) should be necessarily used instead of the power splitter. AWGR is suitable for a wavelength-division-type optical network, but is not suitable for a passive optical network (PON) using a single upstream wavelength. The above-mentioned methods enable a plurality of optical network units to use a common network in realizing an optical network, but do not consider the monitoring of the common network and the monitoring of distortion by the central office.